Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relatively brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
To overcome the problems associated with molding ceramic compositions into products, various alternatives have been suggested. For example, it is believed that the process of manufacturing ceramic articles from metal-containing polymers has the potential to overcome the problems associated with molding and sintering inorganic ceramic compositions. Thus, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See for example, "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol 62, No. 8, pages 893-915 (1983), and the references cited therein. Typically, the organosilicon preceramic polymers are pyrolyzed in an inert gas to form silicon carbide and/or silicon nitride-containing articles, especially fibers. It is believed that the formation of silicon carbide fibers is the only commercial product formed by this technology. Accordingly, there is a need to develop ceramic articles from other metal-containing polymers, especially ones that exhibit superior resistance to high temperature oxidation.
Another process for producing ceramic articles, including fibers, is disclosed in U.S. Pat. Nos. 3,399,979 and 3,403,008. According to these patents, a preformed organic polymeric material is impregnated with a solution of a metal compound, the impregnated material heated to leave a carbonaceous relic containing the metal in finely dispersed form and further heated at 1,000.degree.-2,000.degree. C. in a nonoxidizing atmosphere to form the metal carbide or metal nitride depending on the atmosphere utilized. A similar approach has been taken in the formation of metal oxide fibers. Thus, as disclosed in U.S. Pat. Nos. 3,846,527 and 4,010,233 metal salts are incorporated into polymeric spinning solutions, the solutions spun into fibers, and the fibers calcined in air to yield metal oxide fibers. Use of alternative calcination atmospheres leads to the formation of metal carbide or nitride fibers. Use of metal salt mixtures are disclosed as resulting in bimetallic oxide fibers.
Still another approach has been to disperse ceramic powders in a carrier component such as organic liquids including low molecular weight polymers, spin the dispersion into fibers and then sinter the ceramics. An example of this procedure for producing ceramic fibers such as ferrimagnetic spinel fibers is disclosed in U.S. Pat. No. 4,559,191.
U.S. Pat. No. 4,126,652 discloses a process for preparing metal carbide-containing molded products which comprises heating a molded composition comprising at least one powdery metal selected from the group consisting of B, Ti, Si, Zr, Hf, V, Nb, Ta, Mo, W, Cr, Fe, and U and having an average particle size of not more than 50 microns and an acrylonitrile polymer at a temperature of about 200.degree.-400.degree. C., and then calcining the resulting product at a temperature of about 900.degree.-2,500.degree. C. in an inert atmosphere to form the metal carbide. Metal carbide fibers can be formed by the process which involves spinning the mixture of metal and carbon-forming polymer into fiber, heating to render the fibers infusible and then pyrolyzing to yield the metal carbide. The metals may be added together with any conventional calcining aid including metal oxides. One example in the patent describes adding metallic tungsten and metallic silicon to a polyacrylonitrile solution and ultimately forming fiber consisting of tungsten carbide and silicon carbide.
Of the ceramic fibers which have been produced by the above-mentioned processes, it appears that silicon carbide fiber formed from preceramic polymers is the only ceramic fiber to gain market acceptance. However, the metal-containing polymers are typically of low molecular weight and it has been found difficult to maintain a threadline during spinning such ceramic precursors into fiber. It has also been found that trying to spin a polymeric dope containing ceramic particles is quite difficult, in particular, due to the necessity of loading the polymers with high levels of inorganic substances, which high loadings vastly increase the viscosity of the spinning dopes. On the other hand, impregnating polymeric fibers and the like with aqueous solutions of metal salts has been unsatisfactory in view of the small loadings of metals which are obtained in the fiber. The impregnation method, however, has an advantage over forming ceramic fibers from spinning dopes which contain ceramic or metallic particles, since in the impregnation method, the fiber is spun from known fiber-forming organic materials and, thus, there are no spinning and handling problems with regard to the preceramic fiber. However, to make the impregnation method for forming ceramic fibers practical, methods of obtaining higher loadings of the metal into the polymeric substrate must be found.
It is thus one of the objects of the present invention to provide a novel process for producing titanium-containing ceramic articles.
Another and important object of the present invention is to provide an improved process for producing titanium ceramic fibers which contain an increased titanium content.
These and other objects, aspects and advantages, as well as the scope, nature and utility of the present invention, will be apparent from the following description and appended claims.